Until relatively recently, the vast majority of displays were built around cathode ray tube (CRT) technology, in which beams of electrons excite phosphors at the screen end of a glass tube. In a CRT display, the length of the glass tube increases with the width of the screen. As a result CRT displays tend to be large and bulky. In an effort to produce large display screens without the bulkiness of CRT displays, a wide variety of different flat panel display technologies have been developed. Among the most promising of these technologies have been liquid crystal, gas plasma, vacuum fluorescent, electroluminescent, and optical waveguide technologies. Indeed, liquid crystal and gas plasma displays are rapidly overtaking CRT displays for television and computer display applications.
Optical waveguide based display systems offer a promising alternative to liquid crystal and gas plasma display systems. For example, optical waveguide displays can be fabricated using lighter and less expensive materials and components than liquid crystal and gas plasma displays.
Some optical waveguide display systems are formed from an array of optical waveguides that include a series of taps along their lengths. The taps are configured to remove light from the waveguides at the pixel locations of the display. The taps may be scanned sequentially to emit visible images from the display. Light tapping techniques based on electro-optic, thermo-optic, and liquid crystal effects have been proposed.
Recently, a display apparatus has been proposed that includes an array of optical fibers with liquid-filled cores and an array of elongate piezoelectric elements. The piezoelectric elements are wrapped around respective pixel regions of the optical fibers. The piezoelectric elements generate acoustic waves that are focused onto the centers of the optical fibers at the pixel regions to induce cavitation in the liquid filled cores. The bubbles that are produced by the cavitation scatter light out of the liquid-filled cores to produce visible light at the pixel locations. In this approach, the acoustic waves only propagate in the optical fibers. In general, acoustic waves cannot be focused onto regions that are larger than the acoustic wavelength. Therefore, in order to achieve any type of acoustic wave focusing in this display approach, the acoustic wavelength should be no greater than the optical fiber diameter. The optical fibers in this display approach are 200–300 μm (micrometers) in diameter, in which case the lowest acoustic frequency is on the order of 5 MHz, assuming the optical fibers are filled with water. The acoustic power needed for cavitation (and the associated operating temperature) increases exponentially with acoustic frequency. Therefore, it is desirable to reduce the operating acoustic frequencies in such optical waveguide display systems.